1. Field of the Invention
The present invention relates generally to scanning electron microscopy. More particularly, it relates to scanning electron microscopy used for specimen inspection.
2. Description of the Background Art
An example of a scanning electron microscope (SEM) system is shown in FIG. 1A for purposes of background explanation. The particular system of FIG. 1A is described in U.S. Pat. No. 5,578,821, entitled xe2x80x9cElectron Beam Inspection System and Method,xe2x80x9d issued to Meisberger et al. and assigned to KLA-Tencor Corporation of San Jose, Calif. The disclosure of U.S. Pat. No. 5,578,821 (the Meisberger patent) is hereby incorporated by reference.
FIG. 1A (corresponding to FIG. 5 in the Meisberger patent) is a simplified schematic representation of the paths of the primary, secondary, back-scatter and transmitted electrons through the electron optical column and collection system for electron beam inspection. In brief, FIG. 1A shows a schematic diagram of the various electron beam paths within the column and below substrate 57. Electrons are emitted radially from field emission cathode 81 and appear to originate from a very small bright point source. Under the combined action of the accelerating field and condenser lens magnetic field, the beam is collimated into a parallel beam. Gun anode aperture 87 masks off electrons emitted at unusable angles, while the remaining beam continues on to beam limiting aperture 99. An upper deflector (not depicted) is used for stigmation and alignment, ensuring that the final beam is round and that it passes through the center of the objective lens 104 comprising elements 105, 106 and 107. A condenser lens (not depicted) is mechanically centered to the axis defined by cathode 81 and beam limiting aperture 99. The deflection follows the path shown, so that the scanned, focused probe (beam at point of impact with the substrate) emerges from the objective lens 104.
In High Voltage mode operation, Wien filter deflectors 112 and 113 deflect the secondary electron beam 167 into detector 117. When partially transparent masks are imaged, the transmitted beam 108 passes through electrode system 123 and 124 that spreads the beam 108 before it hits the detector 129. In Low Voltage mode operation, the secondary electron beam is directed by stronger Wien filter deflections toward the low voltage secondary electron detector 160 that may be the same detector used for backscatter imaging at high voltage. Further detail on the system and its operation is described in the Meisberger patent.
FIG. 1B is a diagram illustrating conventional raster electron beam scanning by a scanning electron microscope. As shown, an electron beam spot 152 is scanned 154 over a specimen 20 (for example, a semiconductor wafer) or a portion of a specimen. In the example illustrated, the raster pattern is a zig-zag pattern in the plane of the specimen (the x-y plane). Consider a spot with an effective size S, and an area needing to be scanned of length X in the x-dimension. In that case, X/S (X divided by S) rows would in principle need to be scanned to cover that area. The resolution of the SEM depends on the effective size of the spot.
Unfortunately, conventional SEM systems have their limitations. In particular, as feature sizes in semiconductor circuits continue to shrink, wafer inspection systems need to scan at higher and higher resolutions. For example, recent semiconductor manufacturing processes have 0.18 micron, 0.15 micron, and 0.13 micron linewidths. Future processes will have even smaller linewidths.
The need for such higher resolutions implies the need for smaller spot dimensions of the electron beam as it impinges upon the wafer. The smaller the spot size, the higher the resolution. Submicron spot sizes (for example, 0.5 micron, 0.2 micron, 0.15 micron, 0.1 micron, 0.05 micron, or less) are desirable to inspect features or defects of semiconductors.
Such smaller spot sizes require faster scanning speeds in order to keep inspection times per wafer reasonable. For example, recent semiconductor wafers have diameters of 200 mm or 300 mm. Future wafers will be even larger. To provide the smaller spot sizes at higher scanning speeds, higher beam current densities will be required. Higher beam current density produces greater space-charge repulsion between electrons in the beam. This tends to expand the beam, limiting the achievable beam density at the wafer. The beam spot cannot be given arbitrarily higher current density. Wafer inspection systems using conventional SEM technology are thus limited in their speed.
One possible approach to overcome the above-described problem is a projection system, where a large spot rather than a small one is formed at the wafer, and the secondary electrons from this spot are imaged onto a two-dimensional detector. Such an approach is described in U.S. Pat. No. 5,973,323, xe2x80x9cApparatus and Method for Secondary Electron Emission Microscope,xe2x80x9d issued to Adler et. al and assigned to KLA-Tencor Corporation of San Jose, Calif. However, while such systems are workable, they present added complexities in terms of wafer charging control, beam intensity uniformity, and image aberrations.